linux-kernel - Re: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules

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Date:   Wed, 27 Oct 2021 20:31:08 +0800
From:   Lexi Shao <shaolexi@...wei.com>
To:     <james.clark@....com>
CC:     <acme@...nel.org>, <alexander.shishkin@...ux.intel.com>,
        <jolsa@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-perf-users@...r.kernel.org>, <mark.rutland@....com>,
        <mingo@...hat.com>, <namhyung@...nel.org>, <nixiaoming@...wei.com>,
        <peterz@...radead.org>, <qiuxi1@...wei.com>, <shaolexi@...wei.com>,
        <wangbing6@...wei.com>
Subject: Re: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules

On 27/10/2021 18:24, James Clark wrote:
>On 27/10/2021 10:52, Lexi Shao wrote:
>> On ARM machine, kernel symbols from modules can be resolved to $a
>> instead of printing the actual symbol name. Ignore symbols starting with
>> "$" when building kallsyms rbtree.
>> 
>> A sample stacktrace is shown as follows:
>> 
>> c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms])
>> bf4a66d8 $a+0x78 ([test_module])
>> c0a4f5f4 kthread+0x15c ([kernel.kallsyms])
>> c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms])
>> 
>> On ARM machine, $a/$d symbols are used by the compiler to mark the
>> beginning of code/data part in code section. These symbols are filtered
>> out when linking vmlinux(see scripts/kallsyms.c ignored_prefixes), but
>> are left on modules. So there are $a symbols in /proc/kallsyms which
>> share the same addresses with the actual module symbols and confuses perf
>> when resolving symbols.
>
>Hi Lexi,
>
>Is it worth using or re-implementing the entire is_ignored_symbol() function
>from scripts/kallsyms.c? It seems like this change only fixes one occurrence,
>but is_ignored_symbol() has a big list of other cases.
>
>Unless those cases are different?
>
>Thanks
>James

Hi James,

I don't think it's necessary to cover all the cases listed in
is_ignored_symbol(). As long as the symbols are unique and not overlapping
with each other, they should't cause problems in resolving symbols. So most
of the cases in is_ignored_symbol() should be irrelevant.

Lexi

>
>> 
>> After this patch, the module symbol name is printed:
>> 
>> c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms])
>> bf4a66d8 test_func+0x78 ([test_module])
>> c0a4f5f4 kthread+0x15c ([kernel.kallsyms])
>> c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms])
>> 
>> Signed-off-by: Lexi Shao <shaolexi@...wei.com>
>> ---
>>  tools/perf/util/symbol.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
>> index 0fc9a5410739..35116aed74eb 100644
>> --- a/tools/perf/util/symbol.c
>> +++ b/tools/perf/util/symbol.c
>> @@ -702,6 +702,10 @@ static int map__process_kallsym_symbol(void *arg, const char *name,
>>  	if (!symbol_type__filter(type))
>>  		return 0;
>>  
>> +	/* Ignore local symbols for ARM modules */
>> +	if (name[0] == '$')
>> +		return 0;
>> +
>>  	/*
>>  	 * module symbols are not sorted so we add all
>>  	 * symbols, setting length to 0, and rely on
>>
>